1. Field of the Invention
The present invention is directed to an apparatus for ball forming in semiconductor wire bonding, its control, and a wire bonder and, in particular, to a ball forming apparatus in which control parameters for spark discharge are set in digital data.
2. Description of the Related Arts
FIG. 7 is a block diagram showing the construction of a ball forming apparatus in a conventional wire bonder disclosed in Japanese Patent Application Laid-Open No. 57-39055. Designated at 1 in this figure is a wire for bonding, with one end (the lower end) of the wire passing through a capillary 2 downward. Disposed below the wire 1 is an electric torch electrode 3 from which a discharge takes place by applying high tension voltage to the lower end of the wire 1. The end of the wire 1 passes through a clamper 4 and runs upward so that the wire 1 is wound around a wire spool 5. Designated at la is a ball formed at the lower end of the wire 1 as a result of a spark discharge.
The conventional ball forming apparatus further comprises a discharge voltage generator circuit 6 for applying a high-tension voltage (2200 volts or higher, for example) between the electric torch electrode 3 and the lower end of the wire 1, a current setting potentiometer 7a, a discharge current supplying circuit 7 for supplying between the electric torch electrode 3 and the wire 1 a constant discharge current set by the current potentiometer 7a, a discharge time potentiometer 8a, a discharge time control circuit 8 for presetting or adjusting the discharge time of the discharge current supplying circuit 7, a driving circuit 9 for outputting a trigger signal to the discharge voltage generator circuit 6 and the discharge time control circuit 8 in response to the entry of a start signal from a wire bonder controller (not shown), a gap voltage detector circuit 10 for outputting a gap voltage signal S1 by detecting a gap voltage Vg between the electric torch 3 and the lower end of the wire 1, a discharge state determining circuit 11 for determining whether the voltage detected by the gap voltage detector circuit 10 is greater than or smaller than a reference value, and a determination result output circuit 12 for outputting a discharge state signal so that whether the discharge state is normal is presented on the display unit (not shown) of the wire bonder controller depending on the determination result provided by the determination result output circuit 12.
FIG. 8 is an enlarged view showing partially the wire end and the electric torch electrode before a ball forming phase (before spark discharge). FIG. 9 is an enlarged view showing partially the wire end and the electric torch electrode after a ball is formed (after spark discharge).
FIG. 10 is a timing diagram showing the timing of the operation of the conventional wire bonder. In FIG. 10, references A through D show the wire bonding timing of the wire 1 (chiefly, referring to the timing of extending wire to a semiconductor chip), the input timing of the start signal S9, the timing of ball forming, and the output timing of the determination result signal S10, respectively.
The operation of the conventional bonder is now discussed. When the start signal from the unshown wire bonder is input to the driving circuit 9, the driving circuit 9 in turn sends it to both the discharge voltage generator circuit 6 and the discharge time control circuit 8 to drive both circuits. The discharge voltage generator circuit 6 generates a high-tension voltage (2200 volts or higher), causing a spark to take place between the end of the wire 1 and the electric torch electrode 3 2 thereby producing a discharge path. At the initiation of the spark, the discharge time control circuit 8 activates the discharge current supplying circuit 7 and continues operation for a fixed duration preset by the discharge time potentiometer 8a. The operation of the discharge current supplying circuit 7 causes a current, of which the magnitude is determined by the current setting potentiometer 7a, to flow through the discharge path formed between the end of the wire 1 and the electric torch electrode 3. The joule heat resulting from the current flow through the wire 1 for the fixed duration of time fuses the tip of the wire 1, producing a ball 1a thereon.
The discharge state determining circuit 11 determines whether the ball la has been successfully formed, in response to the gap voltage detector circuit 10 that picks up the gap voltage Vg during ball forming. The determination result as the output signal is sent from the determination result output circuit 12 to the wire bonder controller.
The conventional ball forming apparatus described above determines the discharge time and the magnitude of the discharge current for ball forming depending upon the size (diameter) of a required ball 1, a discharge gap length (the separation between the end of the wire 1 and the electric torch electrode 3) and other factors. Since the discharge gap length varies depending on feed rate variations of the wire 1, an operator adjusts in a analogue fashion the discharge time and the magnitude of discharge current by manually setting the discharge time potentiometer 8a and the current potentiometer 7a. Specifically, several useless test bondings are made on a trial and error basis, before the operator finally determines the discharge time and the discharge current based on the operator's own experience. Such a setting suffers from variations from apparatus to apparatus. Manually setting the discharge time potentiometer 8a and the current potentiometer 7a to precise settings is a difficult task, and the task itself is time-consuming and inefficient.